Conventional resonator structures currently being used in microwave filters suffer from various practical and operational limitations including small tuning range, inadequate spurious performance, high complexity and excessive mass. These characteristics are not optimum for use in the field of space communication applications such as satellite communications where mass, volume and electrical performance are of critical importance. The most commonly used prior art resonator structures for microwave filters are shown in FIGS. 1A, 1B and 1C as discussed below. The relative electric field strength is indicated by in the graphs by shading type.
FIG. 1A illustrates the electrical field pattern of a conventional TE01δ mode (puck) resonator 2 that is supported by a platform support 1. Resonator 2 is made from a material with a high dielectric constant (e.g. generally between 20 and 40). Resonator support 1 has a smaller diameter and is made from a material with a low dielectric constant (e.g. generally between 3 and 5). This kind of resonator and support assembly is disclosed in U.S. Pat. No. 5,608,363 to Cameron et al. FIG. 1A shows the electric field strength in the YZ plane for puck resonator 2 located within a metallic cavity 3. As shown, the maximum electric field intensity generated, resides within the resonator 2. The electric field pattern is symmetrical about the Z-axis in a donut shaped pattern, as shown. Puck resonator 2 is used where a quality factor (Q) greater than 8000 is required in the 3.4 to 4.2 GHz communication band, as is the case for space applications. However, the nearest spurious mode for puck resonator 2 operating at 3.42 GHz is too close to the top of the communication band (4.2 GHz). When puck resonators 2 are combined to produce a filter, these spurious modes move even closer to the filter pass-band due to the cumulative effects of irises, probes and tuning screws causing interference with filters centered between 4.0 and 4.2 GHz. Another important disadvantage of puck resonator 2 is that since the electrical field is spread out (as shown in FIG. 1A), tuning screws do not effectively interrupt the electrical field resulting in a small tuning range. Further, when multiple resonators are combined to form a filter, undesired (stray) couplings are generated between non-adjacent resonators and require additional diagonal probes for cancellation purposes. These diagonal probes result in added complexity, increased mass and performance degradation for the resonator and filter assembly.
FIG. 1B illustrates the electrical field pattern of another conventional type of resonator 5, namely the metal combline (TEM mode) resonator 5. Combline resonator 5 is housed within and is in electrical contact at one end with a metallic cavity 6. Typically, the resonator 5 and metallic cavity 6 are fastened together using mechanical means (i.e. a screw). This structure is commonly used within ground station filters where quality factor (Q) is traded off for reduced mass, size and complexity. Combline resonator 5 exhibits the best spurious performance where the nearest spurious mode is generally greater than two times the fundamental frequency. The size is approximately half of the size of the puck resonator but the resulting quality factor (Q) is generally about half of the Q of the puck resonator. This lower Q makes the metal combline unusable for satellite multiplexer filters. The electric field strength is minimum at the bottom of the resonator and maximum at the top giving a one quarter wave variation over the length of the resonator. A tuning screw (not shown) is placed at the top of metallic cavity 6 where the electric field is strongest, resulting in a large tuning range. The electric field pattern is symmetrical about the Z-axis with no electric field inside the metal resonator. The complexity of the metal combline resonator 5 is less than that of the puck resonator 2 (FIG. 1A) since a supporting platform is not required.
FIG. 1C illustrates the electrical field pattern of a quarter wave dielectric (QWD) resonator 8 operating in the TM01 mode. As shown, QWD resonator 8 is housed within and is in electrical contact at one end with a metallic cavity 9. Typically, QWD resonator 8 and metallic cavity 9 are fastened together using adhesive and/or mechanical means. While, quarter wave dielectric resonator 8 has an improved (i.e. higher) quality factor (Q) in respect of the metal combline resonator 5, QWD resonator 8 still cannot meet the required Q>8000 criteria. This is primarily due to the fact that the quality factor (Q) of QWD resonator 8 is limited due to losses caused by the resonator 8 and cavity 9 being in electrical contact. The electric field strength is minimum at the bottom of the resonator and maximum at the top giving a one quarter wave variation over the length of the resonator. The tuning screw is placed at the top where the electric field is strongest resulting in a large tuning range. The electric field pattern is symmetrical about the Z-axis with some electric field inside the resonator. Due to the electrical and magnetic characteristics associated with QWD resonator 8, a high intensity magnetic field will be produced at one end resulting in high current density in the walls of cavity 9 reducing the quality factor (Q). Again, the QWD resonator 8 is less complex than puck resonator 2 since the supporting platform is not required.